RELATED: Pelosi announces Trump impeachment inquiry as president says he'll release transcript with Ukraine's Zelenskiy
The probe focuses partly on whether Trump abused his presidential powers and sought help from a foreign government to undermine Democratic foe Joe Biden and help his own reelection. Pelosi said such actions would mark a "betrayal of his oath of office" and declared, "No one is above the law."
The impeachment inquiry, after months of investigations by House Democrats of the Trump administration, sets up the party's most direct and consequential confrontation with the president, injects deep uncertainty into the 2020 election campaign and tests anew the nation's constitutional system of checks and balances.
Trump, who thrives on combat, has all but dared Democrats to take this step, confident that the specter of impeachment led by the opposition party will bolster rather than diminish his political support.
Meeting with world leaders at the United Nations, he previewed his defense in an all-caps tweet: "PRESIDENTIAL HARASSMENT!"
RELATED: What is an impeachment inquiry? A look at the impeachment process
U.S. Senator Dick Durbin (D-IL) joined the call for the House to begin a formal impeachment inquiry Tuesday.
Today, I am calling for the initiation of a formal impeachment inquiry by the House of Representatives of President Donald Trump.— Senator Dick Durbin (@SenatorDurbin) September 24, 2019
In a speech on the Senate floor, Durbin called on Congressional Republicans to speak up.
"If this President of the United States can attempt to extort a foreign leader to withhold security funds that were to be given by the United States to this country in order to pursue and promote his own political agenda, we have reached a new low in the United States," Durbin said. "The whistleblower's claim needs to be released to the appropriate congressional committees and evaluated according to the law, and Congressional Republicans - House and Senate - need to make it clear, once and for all, that no President can solicit or strong-arm a foreign country to further his own campaign."
RELATED: Pelosi to pursue Trump impeachment as president says he'll release call transcript
Durbin also called for the whistleblower complaint to be provided to the appropriate House and Senate committees.
Senator Tammy Duckworth (D-IL) joined Durbin in his support for the inquiry.
"Federal law is clear," Duckworth said in a statement. "When an independent Inspector General determines that a whistleblower complaint is both credible and a matter of 'urgent concern' -- the whistleblower complaint must be sent to Congress. Period. Donald Trump's administration violated that law. The House of Representatives should use its constitutional authorities to investigate this cover up and swiftly decide if impeachment is warranted.
Rep. Sean Casten (D-IL 6th District) released a statement on the inquiry, saying in part, "Donald Trump has risked our national security and invited yet another foreign government to intercede to influence our elections. Donald Trump is so singularly focused on promoting his own fortunes - political and otherwise - that he is incapable of defending the Constitutional rights and respecting the will of the American people. His behavior is a clear and present danger to our Republic. I applaud Speaker Pelosi's decision to open an impeachment inquiry into the President of the United States."
Rep. Mike Quigly (D-IL 5th District) also released a statement of support, saying in part, "Congress is now fulfilling our Constitutional responsibility to hold the President accountable for his actions. Today's announcement is a reminder that no one is above the law. I look forward to making sure that the American people get the answers they deserve."
Rep. Jesus "Chuy" Garcia (IL-4th District) said in part in a statement, "President Trump has a pattern of stonewalling congressional investigations, lying, and obstructing justice to cover up his corrupt actions. Our president should not cut deals with foreign governments for his own personal gain. It is our job as Members of Congress, Democrats and Republicans, to protect the votes and the voices of the American people. Our democracy is not for sale."
Rep. Adam Kinzinger (R-IL 16th District) reserved judgement on impeachment for now, but released a statement that said in part, "no foreign government should have an influence on our election, no matter what."
"It is deeply concerning to see Democrats jump ahead of the facts to satiate their desire for impeachment proceedings to begin," the statement from Kinzinger continued.
WATCH: Joe Biden addresses phone transcripts and impeachment inquiry of President Trump
Pelosi's brief statement, delivered without dramatic flourish but in the framework of a constitutional crisis, capped a frenetic weeklong stretch on Capitol Hill as details of a classified whistleblower complaint about Trump burst into the open and momentum shifted toward an impeachment probe.
For months, the Democratic leader has tried calming the push for impeachment, saying the House must investigate the facts and let the public decide. The new drive was led by a group of moderate Democratic lawmakers from political swing districts, many of them with national security backgrounds and serving in Congress for the first time. The freshmen, who largely represent districts previously held by Republicans where Trump is popular, risk their own reelections but say they could no longer stand idle. Amplifying their call were longtime leaders, including Rep. John Lewis of Georgia, the civil rights icon often considered the conscience of House Democrats.
"Now is the time to act," said Lewis, in an address to the House. "To delay or to do otherwise would betray the foundation of our democracy."
The Trump-Ukraine phone call is part of the whistleblower's complaint, though the administration has blocked Congress from getting other details of the report, citing presidential privilege. Trump has authorized the release of a transcript of the call, which is to be made public Wednesday.
"You will see it was a very friendly and totally appropriate call," Trump said.
I am currently at the United Nations representing our Country, but have authorized the release tomorrow of the complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine....— Donald J. Trump (@realDonaldTrump) September 24, 2019
....You will see it was a very friendly and totally appropriate call. No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!— Donald J. Trump (@realDonaldTrump) September 24, 2019
The Associated Press contributed to this report.